1. Field of the Invention
The invention disclosed herein relates to the analysis of underground earth formations and, in particular, to the determination of resistivity of a formation.
2. Description of the Related Art
Exploration and production of hydrocarbons requires precision measurements of geologic formations. Many of the measurements are performed from a borehole penetrating the earth. Measurements of a particular formation provide data to geophysicists for constructing a model of the formation. The model in turn enables a drilling engineer to accurately drill another nearby borehole for exploration or production. For example, the model together with technology to steer a drill bit (i.e., “geosteering”) allow the drilling engineer to accurately navigate through formations avoiding trouble spots to reach an ideal location in a target formation.
Because there are many types of formations and subsurface materials, distinguishing between the different types can be challenging. Thus, to avoid wasting drilling resources, the measurements must be precise.
Taking these measurements in a borehole is generally referred to as “well logging.” One type of well logging involves measuring the resistivity of a formation. The resistivity then can be related to the composition of the formation. A resistivity logging instrument can measure resistivity by conducting a current through the formation using electrodes. Another type of logging instrument, an induction logging instrument, measures resistivity by inducing alternating current loops in the formation. The induction instrument then measures current induced in a receiver coil caused by an alternating magnetic field resulting from the current loops.
Because the induction logging instrument generally has a greater depth of investigation, the induction logging instrument is usually a primary choice in environments where there is sufficient resistivity contrast between a zone of interest and an adjacent zone. An induction logging instrument can be a normal propagation resistivity instrument or an azimuthal resistivity instrument. The normal propagation resistivity instrument has azimuthal symmetry (i.e., does not measure direction associated with a resistivity measurement). The azimuthal resistivity instrument measures a direction associated with a resistivity measurement. Generally, the normal propagation instrument has a greater distance of measurement from the borehole than the azimuthal resistivity instrument.
Unfortunately, the measurements from the azimuthal resistivity instrument may include a constant background signal resulting from a resistivity gradient. The background signal can be as much as ten times or greater than a normal noise floor of the instrument. The background signal can limit the distance from the borehole at which the azimuthal resistivity instrument can measure resistivity. In addition, the background signal can decrease the accuracy of the direction measured with respect to the resistivity measurement.
Therefore, what are needed are techniques for increasing the sensitivity of measurements performed by an induction logging instrument disposed in a borehole. Preferably, the techniques can be applied to an azimuthal resistivity instrument to increase a depth of measurement and increase the accuracy of directional measurement.